1. Field of the Invention
The invention relates to the removal of residual product from a vessel and more specifically to the removal of residual product from a filter-dryer unit employing a mechanical agitator.
2. Background Art
Filter-dryers are used in the production of a variety of materials, including materials such as used in the formation of pharmaceutical powdered medicines. The basic material from which the powder is formed is typically formulated as a slurry of a predefined mixture of crystals suspended in liquid. A batch of the slurry is commonly loaded into filter-dryer apparatus constructed to separate the crystals from the liquid. The filter-dryer unit may be in the form of a sealed, cylindrically shaped vessel having a filter media in its base to allow the liquid to be drained from the vessel. A side discharge or bottom discharge opening is typically provided to allow the resulting powdered product to be moved out of the vessel to a hopper or the like. A mechanical agitator is rotated within the vessel at a relatively slow rate and is typically moved vertically in the slurry material, as well as being rotated, to facilitate drying. The rate of drying may be increased by the application of heat and removal of vapors, in a conventional manner. The agitator also serves to mechanically move the dried product out of the vessel discharge opening.
A problem is that a residue of the finished product tends to remain in various parts of the vessel. The major volume of residual material, called the heel, is left in the lower wall of the vessel and is not readily removable. The lower wall of the vessel is provided with relatively fragile and expensive filter screens which allow the liquid to pass from the vessel while preventing the loss of solids. The mechanical agitator is a large mechanical device which may be on the order of 10 feet in diameter. To ensure that the screens are not damaged by the agitator, a clearance is required between the agitator and the screens, this clearance may be on the order of 10 millimeters. The residue in that space and elsewhere in the vessel is typically washed away at the end of the batch processing. In certain products, such as pharmaceuticals, the batches cannot be mixed and therefore the residue is essentially lost. The lost residue may be as much as 15% of the batch. The cost of such loss in expensive compounds, such as the pharmaceutical compounds, is indeed very high.